The invention relates to a device for scanning a yarn that is moved in its longitudinal direction in a measuring gap with a light beam from a light source, which device has a receiver for light reflected by the yarn and a unit for processing electrical signals from the receiver.
Such a device is known, for example, from EP 0 761 585, in which there is provided a light source which emits light which on the one hand is reflected and on the other hand is also shaded by the yarn. The light received by receivers is converted in a manner known per se into electrical signals for which ranges or threshold values can be provided in order, for example, to detect foreign bodies in the yarn.
This device has the disadvantage that, for example, foreign bodies in the yarn that have the same colour as the emitted light cannot be detected therewith. However, the same is also true, for example, of transparent foreign bodies such as pieces of plastics film in the yarn, so that such foreign bodies cannot be detected with this device.
A further device of this type is known from WO 95/29396, in which three different light sources are associated with a receiver for white light. These three light sources are in the form of three light-emitting diodes, each diode emitting light of a different main wavelength in the visible range. When viewed in the longitudinal direction of the yarn, the light sources are arranged one behind the other next to the yarn and are so oriented that they illuminate the yarn and a background that is located behind the yarn and absorbs as much light as possible. The signals produced in the receiver by reflection of the light by the yarn are processed in such a manner that ratios are formed from the values determined for the signals in the individual main wavelengths, which ratios can in turn be measured against criteria.
A particular disadvantage of this device is that it is only suitable for the evaluation of reflected light with yarn in front of a preferably black background. Moreover, the arrangement of three or more diodes at intervals along the yarn requires a large amount of space, which may not even be available at those locations in textile machines that are intended for such devices. This device has the additional problem that operation of the individual light sources must take into account the movement of the yarn and it must therefore be provided that the same point on the yarn is always illuminated with light of a different colour.